Fix race condition in Effects

Main problem was in AudioFlinger::EffectChain::EffectChain(), where
the thread* argument could possibly have been destroyed before the
creation of the weak pointer
(AudioFlinger::EffectChain::EffectCallback::mThread) around it.

While here, cleaned up a bunch of other pointer-related operations
in order to avoid trafficking in raw pointers to RefBase subclasses.

Bug: 147770363
Change-Id: I508ca94dd38a04e13f1a1c413f548001b961c721